Ch. 10 Acid-Base Titration

Transcription

1 Ch. 10 Acid-Base Titration ~ titration curve ( 적정곡선 ) : ph as a function of added titrant volume Titration of Strong Acid with Strong Base ( 적정시약 ) H + (Strong Acid) + OH - (strong Base) H 2 O K = 1/K w = 1 x Reaction goes to completion 50.0 ml of M KOH with M HBr Starting Point KOH K + + OH poh = - log (0.0200) = 1.70 ph = = ml 0.10 M HBr M KOH Region 1: Before the Equivalence Point ~ ph determined by the excess OH - and dilution ~ V HBr = 3.0 ml OH - + H + H 2 O x mmol in 53.0 ml x3.0 poh = - log [0.70/(53.0)] = - log (0.0132) = 1.88 ph = = 12.12

2 10-1. Titration of Strong Acid with Strong Base 50.0 ml of M KOH with M HBr Region 1: Before the Equivalence Point ~ V HBr = 9.5 ml OH - + H + H 2 O x mmol in ml x9.5 poh = - log [0.05/(59.5)] = - log ( ) = 3.01 ph = = Region 2: At the Equivalence Point ~ ph determined by the dissociation of water ~ V HBr = 10.0 ml OH - + H + H 2 O x x10.0 H 2 O H + + OH - +x +x ph = poh = - log (1.0 x 10-7 ) = 7.0

3 - log scale Titration of Strong Acid with Strong Base 50.0 ml of M KOH with M HBr Region 3: After the Equivalence Point ~ ph determined by the excess H + ~ V HBr = 10.5 ml OH - + H + H 2 O x x mmol in ml ph = - log (0.050/(60.5)) = - log (8.26x10-4 ) = 3.08 OH - + H + H 2 O (e.g.) ph? M NaOH ml M HClO ml OH - + H + H 2 O x x mmol in ml ph = - log (0.553/(37.74)) = - log (0.0147) = 1.83

4 10-1. Titration of Strong Acid with Strong Base

5 10-2. Titration of Weak Acid with Strong Base 50.0 ml of M MES (pk a = 6.27) with M NaOH Region 1: Before Base is Added HA H + + A - K 0.02-x +x +x [H ][A ] x x x a [HA] F- x x x = [H + ] = 1.03 x 10-4 ph = - log (1.03 x 10-4 ) = 3.99 Region 2: Before the Equivalence Point HA + OH - A - + H 2 O : K = 1/K b = 5.4 x 10 7 remaining HA (weak acid) + produced A - (conjugate base) buffer!! ~ V NaOH = 3.0 ml HA + OH - A - + H 2 O 0.020x x ph = pk a + log [A - ]/[HA] = log (0.30/0.70) = 5.90

6 10-2. Titration of Weak Acid with Strong Base 50.0 ml of M MES (pk a = 6.27) with M NaOH Region 2: Before the Equivalence Point HA + OH - A - + H 2 O remaining HA (weak acid) + produced A - (conjugate base) buffer!! ~ V NaOH = 5.0 ml HA + OH - A - + H 2 O 0.020x x ph = pk a + log [A - ]/[HA] = log (0.50/0.50) = 6.27 Region 3: At the Equivalence Point HA + OH - A - + H 2 O contains only A x x x10 A - + H 2 O HA + OH - K b = K w /K a = 1.86x mmol in 60.0 ml 0 0 (1.0/60) x +x +x [HA][OH ] x x x Kb [A ] x x x = [OH - ] = 1.76x10-5 M poh = - log (1.76x10-5 ) = 4.75 ph = =

7 10-2. Titration of Weak Acid with Strong Base 50.0 ml of M MES (pk a = 6.27) with M NaOH Region 4: After the Equivalence Point ~ ph determined by the excess OH - ~ V HBr = ml HA + OH - A - + H 2 O x x mmol in ml [OH - ] = (0.010/60.10) = 1.66x10-4 M poh = - log (1.66 x 10-4 ) = 3.78 ph = = 10.22

8 10-2. Titration of Weak Acid with Strong Base HA + OH - A - + H 2 O (2) buffer ph pk a [A ] log [HA] (4) excess OH - (3) dissociation of A - A - + H 2 O HA + OH - : K b ph(equiv. point) > 7.0 (1) dissociation of HA HA H + + A - : K a

9 10-2. Titration of Weak Acid with Strong Base HPr = Propionic Acid

10 10-3. Titration of Weak Base with Strong Acid ~ reverse of the titration of a weak acid with a strong base B (weak base) + H + (strong acid) BH + 1) Region 1: Before Acid is Added B + H 2 O BH + + OH - K b F-x +x +x 2) Region 2: Before the Equivalence Point B + H + BH + buffer ph = pk a (for BH + ) + log ([B]/[BH + ]) ph = pk a at ½ equivalence point 3) Region 3: At the Equivalence Point B + H + BH + BH + B + H + K a F -x +x +x ph (equivalence point) < 7.0 4) Region 4: After the Equivalence Point B + H + BH + excess

11 10-3. Titration of Weak Base with Strong Acid [e.g.] Titration of 25.0 ml of M pyridine (K b = 1.69x10-9 ) with M HCl Find ph when V HCl = 4.63 ml? B + H + BH x x mmol mmol mmol Before equivalence point! buffer solution (weak base [B] & its conjugate base [BH + ]) 14 [B] ph pka log log 9 [BH ] log pk log a K K w b

12 10-3. Titration of Weak Base with Strong Acid

13 10-2. Acid-Base Titration Curves ph(equiv.) = 7.0 ph(equiv.) < 7.0 [e.g] NaOH + HCl [e.g] NH 3 + HCl ph(equiv.) > 7.0 ph(equiv.) ~ 7.0 [e.g] NaOH + CH 3 COOH [e.g] NH 3 + CH 3 COOH

14 10-4. Finding the End Point Equivalence point ( 당량점 ) : stoichiometry of the reaction End point ( 종말점 ) : abrupt change of a physical property (such as ph) that we measure to locate the equivalence point ~ indicators ( 지시약 ) and ph measurement in acid-base titration Using Indicators to Find the End Point HIn H + + In - K HIn [H ][In [HIn] ] ph pk HIn [In ] log pk [HIn] HIn log Range of Color Change : ph pk HIn - 1 (HIn) ~ pk HIn + 1 (In - ) 1 10 ~ 10 1 In - pk HIn + 1 pk HIn pk HIn - 1 HIn Indicator error : the difference between equivalence point and end point ~ indicator itself an acid or base indicator reacts with analyte or titrant never use more than a few drops of diluted indicator solution a few tenths percent solution (wt/vol) : 2 ~ 3 drops of indicator a few % 10

15 10-4. Finding the End Point

16 10-4. Finding the End Point Choosing an Indicator ~ indicator s color transition range overlaps the steepest part of the titration curve as closely as possible Titration: ml of M Base (pk b = 5.0) with M HCl ph =2 ph (equivalence point) = pk HIn

17 10-4. Finding the End Point Find adequate indicators using Table 8-3 (p185)? analyte weak acid strong acid weak base titrant strong base strong base strong acid ph (equivalence point) indicator > 7.0 pk HIn : 8~ pk HIn ~ 7 < 7.0 pk HIn : 4~6

18 10-4. Finding the End Point Using a ph Electrode to Find the End Point H 6 A + NaOH ph V ( ph/ V) V

19 10-5. Practical Notes (primary standard) ( KHP) HCl, NaOH, KOH : no side reactions and easy to handle but not a primary standard because HCl : 36 ~ 37 %, NaOH, KOH : (carbonate + absorbed water) from the air need standardization : HCl with Na 2 CO 3, (NaOH & KOH) with KHP Strong basic solution attacks glass [(e.g.) buret] plastic bottle

20 10-5. Practical Notes (1 차표준물질 ) ( KHP) HCl, NaOH, KOH : no side reactions and easy to handle but not a primary standard HCl : 36 ~ 37 %, NaOH, KOH : (carbonate + absorbed water) from the air need standardization : HCl with Na 2 CO 3, (NaOH & KOH) with KHP Strong basic solution attacks glass [(e.g.) buret] plastic bottle

21 10-6. Kjeldahl Nitrogen Analysis ~ one of the most widely used methods for determining N in organics such as protein, cereal, flour boiling 1) Kjeldahl digestion: organic C, H, N NH CO 2 + H 2 O H 2 SO 4 (Hg, Cu, Se) compounds catalyze the digestion process. 2) Neutralization of NH 4 + : NH OH - NH 3 (g) + H 2 O 3) Distillation of NH 3 into standard HCl: NH 3 + H + NH 4 + 4) Titration of unreacted HCl with NaOH: H + + OH - H 2 O (e.g.) a typical protein contains 16.2 wt % N solution 0.50 ml solution distilled into 10.0 ml of M HCl unreacted HCl required 3.26 ml of M NaOH Find concentration of protein (mg protein/ml)? mmol NH 3 = mmol of total HCl - mmol of unreacted HCl (= mmol of NaOH) = ( x 10.0) - ( x 3.26) = mmol N mg N = mmol x mg/mmol = mg N mg protein = mg N/(0.162 mg N/mg protein) mg protein/ml = 12.9 mg protein/0.50 ml = 25.8

22 10-7. Putting Your Spreadsheet to Work Charge Balance ~ sum of positive charges must equal to sum of negative charges in any solution HCl + NaOH H + + Cl - + Na + + OH - [H + ] + [Na+] = [Cl - ] + [OH - ] Titration Curve Depends on 1) pk a or acid/base strength 2) acid/base concentration ph ~ 2 ~ Not practical to titrate an acid or a base 1) when it is too weak (0.10 M and pk a < 8) 2) too dilute solution for weak acid/base The weaker the acid, the smaller the break and the more alkaline the equivalence point. Visual indicators can be used for K a of A ph meter provides better precision for weaker acids.

23 10-2. Titration of Weak Acid with Strong Base Titration Curve Depends on 1) pk a or acid/base strength 2) acid/base concentration 0.1 M At equivalence point : CH 3 COO - + H 2 O CH 3 COOH + OH- : K b F-x +x +x 0.01 M M 1) The buffer regions are about the same for all concentrations. CH 3 COOH + OH - CH 3 COO - + H 2 O 2) The equivalence point ph increases with increasing concentration. [Fig.] Dependence of titration curve of 100 ml acetic acid with NaOH. NaOH concentration the same as HOAc concentration

24 10-2. Titration of Weak Acid with Strong Base Titration Curve Depends on 1) pk a or acid/base strength 2) acid/base concentration ~ Not practical to titrate an acid or a base 1) when it is too weak (0.10 M and pk a < 8) 2) too dilute solution for weak acid/base Titration curves Depends on acid concentration : maximum slope decreases with lower acid concentration ~ Equivalence point becomes more difficult to identify Eventually can not titrate acid at very low concentrations

25 Chemistry in Action Melamine (1,3,5-Triazine-2,4,6-triamine) problem in 2008? Melamine added baby formula (New York Times ) ~ four infants dead and sickened more than 60,000 babies in China 플라스틱, 접착제, 주방용조리대, 접시류, 화이트보드, 화학비료물탄우유를잡아내려고단백질함량을측정 시아누르산 (cyanuric acid) 와결합하여신장에결석 m.p. 347 C ~ 멜라민분해시생성 LD50 in rats and mice (ingested): (lethal dose) 4.1 g/kg - Melamine Cyanurate 6.0 g/kg - Melamine 7.7 g/kg - Cyanuric acid 해태제과 ( 중국 OEM 방식 ) ' 미사랑카스타드 139 ppm 멜라민검출 ( 한국일보 ) 미국보다엄격한유럽의기준 (0.5mg/kg b.w./day) 유해 - 체중 30kg 어린이가하루 20 개씩평생중국에서사망사고 : 영유아의주식인분유에서 2,500ppm 의멜라민이검출 ( 식약청 )